Most people realize their credit score affects their ability to get approved for credit. A lower than average credit score is a warning sign to lenders that you are more likely than most to be late on your payments or default on a loan altogether. Based on this assumption, lenders will either charge you a higher interest rate to account for your high credit risk, or elect to deny you credit outright.
Many of the people trying to clean up credit are looking for a way to improve their credit score for this reason. They are working to get into a new home, purchase a car, or refinance their existing loans, and they realize that by improving their credit score they will increase their chances of accomplishing these tasks and may end up saving thousands of dollars in interest payments.